Excellent question, it’s actually easy, just perform the vitals every 5 minutes, and then you can pick which drug you are going with next. The practical considerations, and patient preference, and situation can often guide you.
Aspirin decreases mortality in chest pain, and it treats the cause of the pain, so never forget to start with that one.
Then treat the pain!
Generally, it is quicker to access the nitro, and so that is the best one to begin with. It takes time to do the ECG, and start the IV, and so tactically it makes sense to go with nitro first for the first three doses as stated in the protocol. The patient may have taken loads of nitro prior to your arrival, and you may have a partner certified in IV starts, so this may guide you to go with morphine next, although you still have to give a minimum of three doses of nitro as you cannot count the patient nitro dosing. But you may use the effectiveness of nitro overall in this situation to consider morphine.
The patient may be afraid of morphine, or their nitro could be expired. You may choose more nitro in that situation.
Each time whether you are giving 0.4 of nitro, or 2mg of morphine keep taking the vitals every 5 minutes. In this way if they fall out of protocol for nitro with heart rate below 60 as an example you can adjust your plan. Once they fall out of the directive’s vital sign parameters for either drug, then they can no longer receive that specific medication even if the patient’s vitals return to acceptable parameters